While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, Vol. 30, pp. 322–334, 1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often >100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm) between the anode and the cathode. Herein, the term “organic EL element” encompasses the layers between the anode and cathode electrodes. Reducing the thickness lowered the resistance of the organic layer and has enabled devices that operate at much lower voltages. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, therefore, it is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons, referred to as the electron-transporting layer. Recombination of the injected holes and electrons within the organic EL element results in efficient electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by Tang et al [J. Applied Physics, Vol. 65, Pages 3610–3616, 1989]. The light-emitting layer commonly consists of a host material doped with a guest material. Still further, there has been proposed in U.S. Pat. No. 4,769,292 a four-layer EL element comprising a hole-injecting layer (HIL), a hole-transporting layer (HTL), a light-emitting layer (LEL) and an electron transport/injection layer (ETL). These structures have resulted in improved device efficiency.
There have also been proposed EL devices capable of producing polarized electroluminescence for applications as backlights in liquid crystal displays, as disclosed in U.S. Pat. Nos. 6,040,069A1 and 6,489,044B1, and U.S. Patent Application Publications 20020158574A 1 and 20020079831 A1. Specifically, a class of thermotropic nematic polymers known as poly(fluorene)s have been actively pursued for the production of polarized electroluminescence because the constituting fluorene unit possesses high luminescene yield and its rod-like structure favors uniaxial alignment required for polarized light emission. The uniaxial alignment of the conjugated backbone is usually achieved by thermally annealing a poly(fluorene) film deposited on an alignment layer at a sufficiently high temperature and for a sufficiently long duration of time. The alignment layer is generally a polymer film such as polyimide. For OLED applications, an electrically conductive alignment layer such as poly(3,4-diethylene-dioxythiophene): polystyrene sulfonic acid, PEDOT/PSS, has been used successfully. These alignment films are necessarily rubbed in a specific direction to produce the desired alignment properties. Depending on the molecular weight of the polymer, substantial alignment of the conjugated backbone chromophore will require an annealing temperature as high as 200° C. and above, and an annealing time as long as several hours. Although polarized light emission has been demonstrated in these polymeric devices, there is a need to further improve the light generation efficiency in order to expand the general utility, particularly in display applications where high brightness is almost always preferable.
To overcame some of these disadvantages of polymeric materials, oligomeric analogs with a characteristic shorter chain length have been considered for polarized light emission as described by Geng et al in Chem. Mater. 15, 542 (2003).